Glow-worm F4 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F4 fault code mean?
The F4 code means your Glow-Worm boiler has failed to detect a flame during the ignition sequence. When the burner either doesn't light at all or the flame sensor can't confirm combustion has taken place, the boiler's PCB triggers a safety lockout and displays F4. This prevents unburnt gas from building up inside the appliance — a genuine safety concern. Until the root cause is found and resolved, the boiler will refuse to fire. A single reset may clear a one-off hiccup, but if the fault keeps returning, something physical needs attention. Most of the underlying causes require a Gas Safe registered engineer to diagnose and repair safely.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Frozen or blocked condensate pipe Common
In cold weather this is one of the most frequent triggers of an F4 lockout. The condensate pipe — typically a white or grey plastic pipe (21–32 mm diameter) running from the boiler to an outside drain — can freeze solid, causing back-pressure that prevents normal combustion and shuts the boiler down. If the fault appeared overnight during a cold snap, this is the first thing to investigate.
- Low gas pressure or interrupted gas supply Common
If the gas pressure reaching the boiler drops below the minimum required for ignition, the burner simply won't light. This can happen because the gas meter valve is partially closed, there's a supply interruption from the network, or neighbours on the same supply are also affected. Checking whether other gas appliances in the home (hob, gas fire) are working normally is a quick first indicator.
- Faulty or dirty flame sensor (ionisation probe) Common
The flame sensor monitors whether combustion is actually taking place after the ignition spark fires. Carbon deposits and general wear can coat the sensor tip, making it unable to confirm a flame even when one exists. The boiler then assumes ignition has failed and locks out with F4. Cleaning or replacing this sensor is a straightforward job for an engineer.
- Worn or damaged ignition electrode or lead Common
The ignition electrode creates the spark that lights the gas, and the high-voltage lead carries the electrical pulse to it. Either component can crack, corrode or wear over time. A weak or absent spark means the gas never ignites, and F4 is the result. These are relatively inexpensive parts that engineers replace routinely.
- Blocked or sooty burner Sometimes
Debris, soot and scale can accumulate on the burner over time — especially if the boiler hasn't been serviced recently. A partially or fully blocked burner restricts gas flow to the combustion chamber, causing ignition failures. Engineers clean or replace the burner during a service or repair visit.
- Faulty gas valve Sometimes
The gas valve controls the flow of gas into the burner. If it sticks, fails to open fully, or develops an electrical fault, insufficient gas reaches the burner and ignition fails. Gas valve faults are less common but represent a more significant repair in terms of cost and must only be handled by a Gas Safe engineer.
- Combustion fan failure or poor performance Sometimes
Before ignition, the fan must run to purge any residual gases and create the correct air-to-gas ratio for combustion. If the fan is running slowly, intermittently or not at all, the boiler's air-pressure switch won't confirm the pre-purge cycle, and the ignition sequence is aborted. A faulty fan is an engineer-only repair.
- Blocked or restricted flue Sometimes
Combustion gases must be able to exit through the flue. A bird's nest, debris build-up, or a kinked flue terminal can restrict airflow sufficiently to prevent safe ignition. The boiler shuts down as a safety measure. Check the external flue terminal for obvious obstructions, but flue work itself requires an engineer.
- Faulty PCB (main control board) Rare
If all other components test as healthy, the PCB itself may be misreading signals from the ignition circuit or flame sensor and incorrectly reporting an ignition failure. PCB faults are relatively uncommon and are usually only diagnosed once other causes have been ruled out, as the PCB is one of the more expensive components to replace.
How to fix it
- Check your other gas appliances DIY safe
Before doing anything else, try a gas hob ring or another gas appliance in the home. If nothing gas-powered works, the issue is upstream of your boiler — contact your gas supplier or check whether the meter valve (the handle on the pipe next to the meter) is fully open. Do not attempt to force or adjust any gas pipework yourself.
- Look for a frozen condensate pipe DIY safe
If it's below freezing outside, find the condensate pipe — the plastic overflow pipe that exits the boiler through an external wall and leads to a drain. If it feels solid or you can see ice, pour warm (not boiling) water along its length in stages until it thaws. You can also use a warm hot-water bottle held against the pipe. Once thawed, reset the boiler and see if it fires normally.
- Check boiler system pressure DIY safe
Low system pressure can sometimes contribute to ignition issues. Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when cold. If it's below 0.8 bar, top it up via the filling loop (a braided hose or inline valve beneath the boiler) until it reads around 1.2–1.3 bar, then close the valve securely. Consult your boiler manual if you are unsure how to locate the filling loop.
- Reset the boiler (up to twice) DIY safe
On most Glow-Worm models, turn the CH control knob to the reset position (marked with a power symbol), hold for a few seconds, then release and set to your desired temperature. Allow the boiler a full minute to attempt ignition. If it fires and the fault doesn't return within a day or two, the issue may have been a one-off. If it locks out again with F4, do not keep resetting — you need an engineer.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the fault persists after the checks above, a Gas Safe engineer needs to attend. They will carry out a combustion analysis and check the ignition electrode, ignition lead, flame sensor, gas valve, fan and flue in a systematic way. Do not attempt to access internal boiler components, gas pipework, or the PCB yourself — this is illegal for non-Gas Safe individuals and potentially dangerous.
Parts you may need
- Ignition electrode · from £25
- Ignition lead (HT lead) · from £20
- Flame sensor / ionisation probe · from £30
- Combustion fan · from £95
- Gas valve · from £220
- Condensate pipe insulation kit · from £15
- PCB (main control board) · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Glow-Worm F4 fault myself?
Very limited self-help is possible. You can thaw a frozen condensate pipe, check your gas supply is on, verify system pressure and attempt a reset — all described in the steps above. Everything else — the ignition electrode, flame sensor, gas valve, fan and PCB — must be handled by a Gas Safe registered engineer. Attempting to work on gas components without registration is both illegal and dangerous.
Why does my Glow-Worm keep showing F4 every winter?
If F4 appears whenever the temperature drops, a frozen condensate pipe is almost certainly the culprit. The pipe runs outside the insulated envelope of your home and is highly susceptible to freezing. The long-term solution is to have a Gas Safe engineer insulate the exposed section of the pipe properly, which typically costs £50–£100 and prevents the problem from recurring each winter.
How much does it cost to fix a Glow-Worm F4 fault?
For the most common causes — a cleaned or replaced flame sensor, new ignition electrode or lead — most homeowners pay roughly £100–£200 including labour. A fan replacement typically brings the cost to around £200–£300. A gas valve replacement is more involved, with parts and labour usually sitting in the £280–£380 range. PCB replacement can add £150–£300 on top of whatever else needs doing, so if the PCB is at fault on an older boiler it's worth weighing that cost against a new boiler installation.
Is the Glow-Worm F4 code dangerous?
The lockout itself is a safety feature, not a hazard — it stops the boiler running in an unsafe condition. However, the underlying cause can be serious: an ignition failure can allow small amounts of unburnt gas to accumulate, and a fault with the flue or combustion system carries a carbon monoxide risk. Don't attempt repeated resets hoping the fault clears itself, and make sure you have a working carbon monoxide alarm fitted near the boiler while you wait for an engineer.